1. Field of the Invention
The present invention relates generally to an over-power protection circuit, and more specifically to the over-power protection circuit used for a switching mode power converter.
2. Description of Related Art
Various power converters have been widely used to provide a regulated voltage and current. For the sake of safety reasons, an over-power protection means must be provided to protect both the power converter itself and the system it powers. A limited power output is thus required for the power converter during the conditions for overloading, short-circuit and feedback open loop.
FIG. 1 illustrates the topology of a conventional power converter comprising a transformer T1, a driving control unit 14, a driving output unit 10, an oscillator 12 and a switching device Q1. A primary-side switching current Ip of the transformer T1, generates a current sense signal VCS through a current sense resistor RS. The driving control unit 14 receives a feedback signal VFB from the output of the power converter. The driving control unit 14 includes a connection unit 144, a power limit comparator 146 and a PWM comparator 148. An input of the power limit comparator 146 and the PWM comparator 148 connects to the current sense resistor RS to receive the current sense signal VCS. The other input of the power limit comparator 146 connects to a maximum power signal VLMT. The other input of the PWM comparator 148 connects to the output of the power converter to receive the feedback signal VFB. The power limit comparator 146 generates an over current signal OC as the current sense signal VCS is higher than the maximum power signal VLMT. The PWM comparator 148 outputs a feedback control signal CNTR as the current sense signal VCS is higher than the feedback signal VFB. The over current signal OC and the feedback control signal CNTR generate a clear signal CLR via the connection unit 144.
The driving output unit 10 generates a switching signal VPWM to switch a switching device in response to a clock signal CLK of the oscillator 12 and the clear signal CLR of the driving control unit 14.
The feedback control signal CNTR is generated when the signal VCS is higher than the feedback signal VFB. The over current signal OC is generated when the signal VCS is higher than the maximum power signal VLMT. The clear signal CLR is adjusted by the feedback control signal CNTR or the over current signal OC for controlling the driving output unit 10 to periodically disable the switching signal VPWM. Therefore, the output of the power converter is regulated, and the primary switching current and the maximum power are limited further.
However, the power converter of a printer or a scanner has to provide a large output power and torque to the motor as the input power of the power converter starts up in the practical application. Meanwhile, the over-current and over-power occurs and causes the power converter performing an improper protecting action. The traditional power converter often employs an over design for providing a maximum output power to satisfy a motor load needed. In other words, the traditional power converter must provide a peak power and extends the duration to the motor load. Conventionally, the main power switch and the other power devices have to increase the voltage/current stress, so the cost due to the peak power is always higher than the maximum output power. Therefore, it is very important to provide a proper protection and to avoid an erroneous judgment for the PWM controller of the power converter.